Method for limiting amplifier input current to avoid low voltage conditions

ABSTRACT

A predictive brownout prevention system may be configured to prevent brownout of an audio output signal. Particularly, the brownout prevention system may be configured to receive information indicative of an amplitude of the audio input signal, receive information indicative of a condition of the power supply, receive information indicative of one or more of the following: 1) adaptive estimates of power supply conditions; 2) anticipated effects of power supply capacitance; and 3) condition of a load impedance, determine from the received information whether a brownout condition exists, and responsive to determining the brownout condition exists, generate the selectable attenuation signal to reduce an amplitude of the audio output signal such that the signal path attenuates the audio input signal or a derivative thereof in order to prevent brownout prior to propagation to the audio output of a portion of the audio input signal having the brownout condition.

CROSS-REFERENCES AND RELATED APPLICATION

The present disclosure cross references U.S. Provisional PatentApplication Ser. No. 61/834,696, filed Jun. 13, 2013, and U.S.Non-Provisional patent application Ser. No. 14/169,349, filed Jan. 31,2014, which are both incorporated by reference herein in their entirety.

The present disclosure claims benefit of U.S. Provisional PatentApplication Ser. No. 62/348,364, filed Jun. 10, 2016 and U.S.Provisional Patent Application Ser. No. 62/382,844, filed Sep. 2, 2016,which are both incorporated by reference herein in their entirety.

FIELD OF DISCLOSURE

The present disclosure relates in general to circuits for audio devices,including without limitation personal audio devices such as wirelesstelephones and media players, and more specifically, to systems andmethods for predictively preventing a brownout condition in a personalaudio device.

BACKGROUND

Personal audio devices, including wireless telephones, such asmobile/cellular telephones, cordless telephones, mp3 players, and otherconsumer audio devices, are in widespread use. Such personal audiodevices may include circuitry for driving a pair of headphones or one ormore speakers. Such circuitry often includes a power amplifier fordriving an audio output signal to headphones or speakers.

In general, personal audio devices continue to be reduced in size, yetmany users desire louder sound from these personal audio devices. Thisplaces physical size constraints on a battery for powering components ofthe personal audio devices at the same time audio subsystems of suchpersonal audio devices are demanding more output power. With the desirefor higher audio volumes and quality, often a boosted supply voltagehigher than the battery voltage is generated in order to supply an audioamplifier and deliver more power to the speaker load. As more power isdelivered to the speaker load, more strain is placed on the battery of apersonal audio device.

A battery includes an output impedance, and thus heavy loadingconditions on a battery can cause a battery's output voltage to drop.Such drop in output voltage may be more prominent when the battery has alow level of charge. The sudden voltage drop produced by this loadingevent has the potential to reduce the battery's output voltage to apoint where certain subsystems on the device are no longer able tofunction properly. When the battery is in a weakened or lower chargestate and the personal audio device offers no protection against suchweakened or lower charge state, often the end result is the personalaudio device resetting itself due to a low voltage condition. Thisself-reset condition may be displeasing to a user of the personal audiodevice and thus problematic for the provider of the personal audiodevice (e.g., manufacturer, vendor, reseller, or other provider in achain of commerce). Such a condition or conditions similar thereto inwhich an unintentional voltage drop occurs are commonly referred to as“brownout” conditions.

Traditional approaches to mitigation of brownout conditions in personalaudio devices have been reactive in nature, in that a reactive brownoutreduction system typically identifies the occurrence of a batteryvoltage falling below a predetermined voltage threshold (e.g.,configured by a user or a provider of the personal audio device) andreacts responsive to the battery voltage falling below such threshold.An example of such reaction is a reduction of audio volume in order toreduce an audio amplifier's load on the battery.

This reactive methodology is based on a concept that an undesirableevent has already occurred to the battery supply, and thus the personalaudio device quickly takes action to reduce loading in order to preventbrownout of the personal audio device. Subsystems other than the audiosubsystem and powered by the battery supply may also react independentlyin order to reduce loading on the battery supply and allow it to returnto a safe level in order to maintain functionality of more criticalsubsystems of the personal audio device. Such reactive approaches dolittle or nothing to prevent the audio subsystem, and in particular anaudio amplifier, from being a cause of the battery supply falling to anundesirable level that may trigger a brownout condition. A reactivebrownout reduction system typically has no knowledge of the audiocontent and by extension, no knowledge of actual power supply loadingcaused by an audio signal path. Instead, such existing systems typicallyassume that the loading of an output amplifier of the audio signal pathis the source of the supply drop and blindly reduce loading of theoutput amplifier, even if it is not the main source of the reduction inpower supply.

A reactive brownout reduction system requires a certain amount of timeto react before the audio signal to the audio amplifier is attenuated.Once the voltage supply of the battery drops, it also takes anadditional amount of time to attenuate the audio signal and allow thebattery supply to return to a “safe” operating voltage. The cumulativeinitial reaction time, system response time, and the battery supplyrecovery time may cause the system to spend a significant amount of timebelow the preconfigured threshold voltage of the battery supply.

If the audio system, in particular the audio amplifier, is the primarycause of the battery supply drop, and the battery is in a weakenedstate, this reactive methodology also has the potential of getting intoa state of operation where the audio volume is repeatedly attenuated andthen allowed to gain back up. From a user's perspective, this canproduce a “pumping” effect of the audio content, where audio volumerepeatedly gets louder and softer, as the reactive brownout reductionsystem may put the reactive brownout response into a continual loop.

SUMMARY

In accordance with the teachings of the present disclosure, certaindisadvantages and problems associated with loudspeaker electricalidentification have been reduced or eliminated.

In accordance with embodiments of the present disclosure, an apparatusfor providing an audio output signal to an audio transducer may includea signal path comprising an audio input configured to receive an audioinput signal, an audio output configured to provide an audio outputsignal, a power supply input configured to receive a power supplyvoltage, and an attenuation block configured to receive informationindicative of one or more of the following: 1) adaptive estimates ofpower supply conditions; 2) anticipated effects of power supplycapacitance; and 3) at least one condition of a complex load impedance;and in response to determining that a portion of the audio input signalhas reached a maximum power threshold, generate a selectable attenuationsignal to reduce an amplitude of the audio output signal such that thesignal path attenuates the audio input signal or a derivative thereof inorder to prevent brownout prior to propagation to the audio output ofthe portion of the audio input signal.

In accordance with these and other embodiments of the presentdisclosure, a method for providing an audio output signal to an audiotransducer may include receiving information indicative of an amplitudeof an audio input signal, receiving information indicative of acondition of a power supply of a signal path having an audio input forreceiving the audio input signal and an audio output for providing theaudio output signal, receiving information indicative of one or more ofthe following: 1) adaptive estimates of power supply conditions 2)anticipated effects of power supply capacitance; and 3) at least onecondition of a complex load impedance, and in response to determiningthat a portion of the audio input signal has reached a maximum powerthreshold, causing attenuation of the audio input signal or a derivativethereof to reduce an amplitude of the audio output signal in order toprevent brownout prior to propagation to the audio output of the portionof the audio input signal.

Technical advantages of the present disclosure may be readily apparentto one having ordinary skill in the art from the figures, descriptionand claims included herein. The objects and advantages of theembodiments will be realized and achieved at least by the elements,features, and combinations particularly pointed out in the claims.

It is to be understood that both the foregoing general description andthe following detailed description are explanatory examples and are notrestrictive of the claims set forth in this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the example, present embodiments andcertain advantages thereof may be acquired by referring to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numbers indicate like features, and wherein:

FIG. 1 is an illustration of an example personal audio device, inaccordance with embodiments of the present disclosure;

FIG. 2 is a block diagram of selected components of an example audiointegrated circuit of a personal audio device, in accordance withembodiments of the present disclosure;

FIG. 3 is a block diagram of selected components of a predictivebrownout prevention system for use within the audio integrated circuitdepicted in FIG. 2, in accordance with embodiments of the presentdisclosure;

FIG. 4 is a circuit diagram of a model of an example battery, inaccordance with embodiments of the present disclosure;

FIG. 5 is a graph showing an example relationship between outputimpedance and battery charge voltage of the model of the example batterydepicted in FIG. 4, in accordance with embodiments of the presentdisclosure;

FIG. 6 is a graph of an example gain transfer function, in accordancewith embodiments of the present disclosure;

FIG. 7 is a graph of another example gain transfer function, inaccordance with embodiments of the present disclosure;

FIG. 8 is a graphical representation of various signals versus time thatmay be present when a predictive brownout control system triggersattenuation of an audio signal to prevent brownout, in accordance withembodiments of the present disclosure;

FIG. 9 is a graphical representation of various signals versus time thatmay be present when a predictive brownout control system does nottrigger attenuation of an audio signal to prevent brownout, inaccordance with embodiments of the present disclosure;

FIG. 10 is a block diagram of selected components of an example audiointegrated circuit of a personal audio device, in accordance withembodiments of the present disclosure;

FIG. 11 is a plot diagram of power versus time depicting the effects ofbulk capacitance on a chirp signal, in accordance with embodiments ofthe present disclosure;

FIG. 12 is a graphical representation of a bottom-side envelope of asupply voltage, in accordance with embodiments of the presentdisclosure;

FIG. 13 is a graphical representation of an upper-side envelope of asupply voltage, in accordance with embodiments of the presentdisclosure; and

FIG. 14 is a block diagram of an adaptive battery model, in accordancewith embodiments of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 is an illustration of an example personal audio device 1, inaccordance with embodiments of the present disclosure. Personal audiodevice 1 is an example of a device in which techniques in accordancewith embodiments of the present disclosure may be employed, but it isunderstood that not all of the elements or configurations embodied inillustrated personal audio device 1, or in the circuits depicted insubsequent illustrations, are required in order to practice the subjectmatter recited in the claims. Personal audio device 1 may include atransducer such as speaker 5 that reproduces distant speech received bypersonal audio device 1, along with other local audio events such asringtones, stored audio program material, injection of near-end speech(i.e., the speech of the user of personal audio device 1) to provide abalanced conversational perception, and other audio that requiresreproduction by personal audio device 1, such as sources from webpagesor other network communications received by personal audio device 1 andaudio indications such as a low battery indication and other systemevent notifications. In addition or alternatively, a headset 3 may becoupled to personal audio device 1 for generating audio. As shown inFIG. 1, a headset 3 may be in the form of a pair of earbud speakers 8Aand 8B. A plug 4 may provide for connection of headset 3 to anelectrical terminal of personal audio device 1. Headset 3 and speaker 5depicted in FIG. 1 are merely examples, and it is understood thatpersonal audio device 1 may be used in connection with a variety ofaudio transducers, including without limitation, captive or integratedspeakers, headphones, earbuds, in-ear earphones, and external speakers.

Personal audio device 1 may provide a display to a user and receive userinput using a touch screen 2, or alternatively, a standard LCD may becombined with various buttons, sliders, and/or dials disposed on theface and/or sides of personal audio device 1. As also shown in FIG. 1,personal audio device 1 may include an audio integrated circuit (IC) 9for generating an analog audio signal for transmission to headset 3,speaker 5, and/or another audio transducer.

FIG. 2 is a block diagram of selected components of an example audio IC9 of a personal audio device, in accordance with embodiments of thepresent disclosure. As shown in FIG. 2, a digital audio source 18 (e.g.,a processor, digital signal processor, microcontroller, test equipment,or other suitable digital audio source) may supply a digital audio inputsignal AUDIO_IN to a predictive brownout prevention system 20, which mayprocess digital audio input signal AUDIO_IN and provide such processedsignal to a digital-to-analog converter (DAC) 14, which may in turnsupply an analog audio input signal V_(IN) to a power amplifier stage A1which may amplify or attenuate the audio input signal V_(IN) and providean audio output signal V_(OUT), which may operate a speaker, headphonetransducer, and/or a line level signal output. Although amplifier A1 isdepicted as a single-ended output generating a single-ended audio outputsignal V_(OUT), in some embodiments, amplifier A1 may comprise adifferential output, and may thus provide a differential audio outputsignal V_(OUT).

A power supply 10 may provide a power supply voltage V_(SUPPLY) to thepower supply rail inputs of amplifier A1. Power supply 10 may comprise acharge pump power supply, a switching direct current-to-direct currentconverter, a linear regulator, or any other suitable power supply.

As discussed in greater detail elsewhere in this disclosure, predictivebrownout prevention system 20 may be configured to prevent brownout ofaudio output signal V_(OUT). As used herein, the term “brownout” broadlyrefers to an unintentional drop of one or more supply voltages withinpersonal audio device 1, which may lead to improper or undesiredoperation of one or more components receiving such one or more supplyvoltages. To carry out this functionality, predictive brownoutprevention system 20 may receive information indicative of an amplitudeof digital audio input signal AUDIO_IN (e.g., by monitoring acharacteristic indicative of an amplitude of digital audio input signalAUDIO_IN). Although many embodiments disclosed herein contemplate suchmonitoring as carried out by directly extracting amplitude informationfrom digital audio input signal AUDIO_IN or a buffered version thereof,in other embodiments, such monitoring may be of any signal derivative ofdigital audio input signal AUDIO_IN (e.g., any signal within the signalpath from digital audio input signal AUDIO_IN to audio output signalV_(OUT)). Predictive brownout prevention system 20 may also receiveinformation indicative of a condition of power supply 10. In someembodiments, the condition of power supply 10 may be indicative of amaximum amplitude of audio output signal V_(OUT) that may be output byamplifier A1 or a supply current consumed by amplifier A1 triggering abrownout condition occurring or violating a user-defined or other typeof threshold indicating a brownout condition. As used throughout thisdisclosure, the term “brownout condition” may broadly refer to acondition or situation in which a brownout may actually occur, or acondition or situation wherein a brownout may potentially occur, basedon parameters measured by predictive brownout prevention system 20, asdescribed in greater detail elsewhere in this disclosure. In these andother embodiments, the condition of power supply 10 may be determined byat least one of power supply voltage V_(SUPPLY), a current of powersupply 10, an internal impedance of power supply 10, impedances externalto power supply 10, and a predicted behavior of power supply 10 inresponse to loading conditions of power supply 10.

Predictive brownout prevention system 20 may determine from the physicalquantity indicative of an amplitude of digital audio input signalAUDIO_IN and the information indicative of the condition of power supply10 whether a brownout condition exists wherein the audio output signalV_(OUT) would brownout responsive to digital audio input signal AUDIO_INin the absence of attenuation within the signal path from digital audioinput signal AUDIO_IN to audio output signal V_(OUT). Responsive todetermining the brownout condition exists, predictive brownoutprevention system 20 may generate a selectable attenuation signal toreduce an amplitude of audio output signal V_(OUT) such that the signalpath attenuates digital audio input signal AUDIO_IN or a derivativethereof in order to prevent brownout prior to propagation to the audiooutput of amplifier A1 of a portion of digital audio input signalAUDIO_IN having the brownout condition. In some embodiments, suchattenuation may include reducing an audio volume of digital audio inputsignal AUDIO_IN or a derivative thereof within the signal path.

In some embodiments, attenuation may include applying a non-linear gainto digital audio input signal AUDIO_IN or a derivative thereof withinthe signal path. In some embodiments, applying a non-linear gain mayinclude clipping digital audio input signal AUDIO_IN or a derivativethereof to a maximum amplitude. For example, such attenuation orclipping may take place in a digital path portion of the signal path(e.g., between digital audio source 18 and DAC 14). Alternatively or inaddition, such attenuation (whether linear or non-linear) or clippingmay take place in an analog path portion of the signal path (e.g.,between DAC 14 and the output node), such as by applying a variable gainto an output stage of DAC 14 and/or a variable gain to amplifier A1.

In these and other embodiments, as described in greater detail below,attenuation may include soft clipping the audio input signal or thederivative thereof with a gain transfer function whose mathematicalderivative is a continuous function. For example, soft clipping may beapplied by an arctangent filter to the audio input signal or thederivative thereof.

FIG. 3 is a block diagram of selected components of an examplepredictive brownout prevention system 20, in accordance with embodimentsof the present disclosure. In the embodiments represented by FIG. 3,predictive brownout prevention system 20 may include an audio amplitudedetection and volume adjust block 110, a power supply monitoring block120, and a predictive control state machine block 140.

User configurations, including audio user configurations 102, supplyuser configurations 106, and/or predictive control user configurations108 may be applied to volume adjust block 110, power supply monitoringblock 120, and predictive control state machine block 140, respectively.Audio user configurations 102 may include, but are not limited to, theability to manipulate audio amplitude detector 116. These userconfigurations may allow the user to set such detection parameters thatinclude, but are not limited to, peak-level thresholds,root-mean-squares-level thresholds, frequencies and/or durations ofconcern, and/or the load impedance on the amplifier. Supply userconfigurations 106 may include, but are not limited to, the ability ofthe user to set various voltage, impedance, current consumption, and/orbehavioral thresholds of the battery supply and/or power behaviorcharacteristics of audio IC 9. These thresholds may allow the user tocustomize when a battery is considered to be in a weakened state ofoperation that may produce a voltage drop when under load. Predictivecontrol user configurations 108 may allow the user the ability tomanipulate the response of the predictive brownout prevention system 20.These may include, but are not limited to, volume adjustments, controldelays, masking or weighting of supply information relative to the audiocontent, and what types and thresholds of audio content to predictivelyattenuate.

User configurability of predictive brownout prevention system 20 may bedesirable as each different design of a portable audio device may havedifferent parameters of concern, including without limitation differentbattery output voltages, different battery characteristics, a differentaudio amplifier, and/or a different audio load. This variation of thesystem requirements and parameters for different personal audio devicesmay dictate that the audio monitoring of amplitude detection and volumeadjust block 110, the supply monitoring of power supply monitoring block120, and control by predictive control state machine block 140 should beflexible, adaptable, and user configurable so that predictive brownoutprevention may be appropriately optimized for each personal audiodevice. While user flexibility to “tune” a response of predictivebrownout prevention system 20 may be desirable in some instances, insome embodiments, some or all of parameters associated with audio userconfigurations 102, supply user configurations 106, and/or predictivecontrol user configurations 108 may be fixed to a specific set of values(e.g., by a provider of a personal audio device).

As shown in FIG. 3, power supply monitoring block 120 may comprisevoltage monitor 122, battery impedance monitor 124, and supply responsepredictor 126. Voltage monitor 122 may be configured to receive powersupply information 104 and perform a comparison of a voltage of abattery for supplying power for power supply 10 against, for example, auser-configurable threshold set within supply user configurations 106. Auser may have the flexibility to determine such voltage threshold andadjust it based on the requirements of the other components withinpersonal audio device 1. In some embodiments, multiple voltagethresholds may be set within supply user configurations 106, which wouldallow predictive control state machine 140 to monitor for and react todifferent levels of predictive audio loading from the audio amplitudedetector 116.

Battery impedance monitor 124 may be configured to receive power supplyinformation 104 and record recent loading conditions and track theeffect of changes in current consumption which may produce correspondingchanges in battery impedance. As a battery becomes “weaker” via itslevel of charge, discharge current, aging of the battery, and/orenvironmental effects, its output impedance may increase. Under no load,the battery's output impedance may have little effect on the battery'soutput voltage. However, the output impedance has a significant impacton the voltage produced on the output terminals of the battery whencurrent is being provided. If power supply 10 comprises a directcurrent-to-direct current converter, such as a boost converter, buckconverter, linear regulator, or charge pump, to regulate the V_(SUPPLY)voltage to the amplifier A1, the direct current-to-direct currentconverter's characteristics may be encompassed as a part of the powersupply information 104, battery impedance monitor 124, or supplyresponse predictor 126.

FIG. 4 is a circuit diagram of a model 40 of an example battery, inaccordance with embodiments of the present disclosure. As shown in FIG.4, a battery may be modeled as having an ideal voltage supply 42outputting a voltage V_(IDEAL) and an output impedance 44 with avariable impedance Z_(OUT) which may vary due to change in level ofbattery charge, discharge current, aging of the battery, and/orenvironmental effects. FIG. 5 is a graph showing an example relationshipbetween output impedance and battery charge voltage of the model 40depicted in FIG. 4, in accordance with embodiments of the presentdisclosure, which shows that variable impedance Z_(OUT) may vary due tochange in level of battery charge. As current I_(LOAD) delivered from abattery increases, the output voltage V_(BATT) it generates (and whichmay be delivered to power supply 10 to provide power to elements ofaudio IC 9) may decrease. Battery impedance monitor 124 may monitor thevariance of this variable impedance Z_(OUT), and if applicable, anadditional impedance external to the battery (e.g., those present onpower supply voltage V_(SUPPLY) to amplifier A1).

Supply response predictor 126 may be configured to receive power supplyinformation 104 and predict future behavior of a battery supply undervarious loading conditions based on monitoring recent behavioral historyof the battery supply. An audio amplifier (e.g., amplifier A1) may nothave enough system-level visibility to be able to determine the totalabsolute loading on the battery supply at any given time. However,supply response predictor 126 may be able to determine what anamplifier's loading contribution is on a battery supply and monitor howthe battery supply responds to the changes in loading produced by theamplifier. Such information enables supply response predictor 126 toestimate how large of a supply voltage drop may occur when a certainamount of current is consumed by amplifier A1. When the status of supplyresponse predictor 126 is combined with status of voltage monitor 122,status of battery impedance monitor 124, and status of audio amplitudedetector 116, predictive control state machine 140 may determine howlarge of an audio output signal V_(OUT) may be produced by amplifier A1without producing a large enough voltage drop in a battery poweringamplifier A1 in order to trigger a brownout condition.

As shown in FIG. 3, audio amplitude detection and volume adjust block110 may comprise an audio buffer 112, volume controller 114, and audioamplitude detector 116, and may be configured to monitor and manipulatethe digital audio input signal AUDIO_IN or a derivative thereof.Portions of audio amplitude detection and volume adjust block 110 (e.g.,audio buffer 112, volume controller 114) may be integral to the signalpath from digital audio input signal AUDIO_IN to audio output signalV_(OUT). In some embodiments, all or a portion of the functionality ofaudio amplitude detection and volume adjust block 110 may be integral toamplifier A1. In these and other embodiments, all or a portion of thefunctionality of audio amplitude detection and volume adjust block 110may be implemented in software or firmware. Thus, one or more featuresof audio amplitude detection and volume adjust block 110 may beimplemented as software or firmware, as one or more separate orintegrated pieces of hardware relative to amplifier A1, or anycombination thereof.

Audio buffer 112 may be any system, device, or apparatus that provides adelay to allow the audio amplitude detector 116 and/or predictivecontrol state machine 140 adequate time to react prior to digital audioinput signal AUDIO_IN propagating through the signal path. For example,audio buffer 112 may provide sufficient delay such that its delay plusthe group delay of the signal path up to volume controller 114 isgreater than the processing time of audio amplitude detector 116,predictive control state machine 140, and volume controller 114. In someembodiments, audio buffer 112 may comprise a memory. In these and otherembodiments, audio buffer 112 may include an intrinsic group delay of anaudio path, delay caused by audio processing, and/or other suitabledelay.

In more robust audio amplifier systems, an audio data path memory bufferis often available as a part of another feature that may also need alook-ahead or some time for pre-processing. When this is the case, thesame memory buffer can be utilized as audio buffer 112 for predictivebrownout prevention as long as it is large enough and has sufficientdelay to allow for processing of other components of predictive brownoutprevention system 20.

In some embodiments, an overall delay within a signal path may besufficiently large enough to allow for processing by components ofpredictive brownout prevention system 20. In such embodiments, audiobuffer 112 may not be present.

In the embodiments represented by FIG. 3, audio amplitude detector 116may monitor digital audio input signal AUDIO_IN entering audio buffer112. In such embodiments, audio amplitude detector 116 may evaluate suchaudio data against one or more thresholds (e.g., set within audio userconfigurations 102) in order to identify any incoming audio signals thatmay produce a loading condition large enough to put a strain on thebattery supply powering power supply 10, producing a voltage drop, andrisking a brownout condition if such audio signal is reproduced by audioamplifier A1. A status determination generated by audio amplitudedetector 116 and provided to predictive control state machine block 140may be based on any number and type of parameters including, but notlimited to, a physical quantity of the audio signal (e.g., frequency,peak amplitude, power, etc.), characteristics of amplifier A1 (e.g.,efficiency), and/or load impedance of an output of amplifier A1.

Although FIG. 3 depicts audio amplitude detector 116 monitoring digitalaudio input signal AUDIO_IN, in other embodiments audio amplitudedetector 116 may detect a derivative of digital audio input signalAUDIO_IN elsewhere within the signal path of audio IC 9.

Volume controller 114 may comprise any system, device, or apparatusconfigured to, based on a volume control signal generated by predictivecontrol state machine 140, control a volume of or otherwise apply aselectable gain to the audio signal buffered by audio buffer 112 (e.g.,prior to communication of the audio signal to DAC 14). Thus, insituations where predictive control state machine 140 determines abrownout condition exists, it may communicate the volume control signal,and in response thereto, volume controller 114 may attenuate the audiosignal propagating through the audio signal path. In some embodiments,the volume controller 114 may attenuate the audio signal by reducing anaudio volume of the audio signal. In these and other embodiments, thevolume controller 114 may attenuate the audio signal in response to abrownout condition by applying a non-linear gain to the audio signal.For example, as shown in FIG. 6, volume controller 114 may apply“hard-clipping” to the audio signal in response to a brownout conditionsuch that a gain transfer function (e.g., ƒ|VOUT(|AUDIO_IN|)|) for theaudio signal may be such that the mathematical derivative of the gaintransfer function includes at least one point of discontinuity. Asanother example, as shown in FIG. 7, volume controller 114 may apply“soft-clipping” to the audio signal in response to a brownout conditionsuch that the gain transfer function for the audio signal may be suchthat the mathematical derivative of the gain transfer function is acontinuous function. Such a soft-clipping gain transfer function may beimplemented in any suitable manner, including by applying an arctangentfilter to the audio signal.

As shown in FIG. 3, predictive control state machine 140 may receivestatus information from audio amplitude detector 116, voltage monitor122, battery impedance monitor 124, and supply response predictor 126and, based on such status information, determine whether to attenuate(e.g., reduce an audio volume via volume controller 114) digital audioinput signal AUDIO_IN (or a derivative thereof) in order to protect froma battery supply voltage drop that might occur if such signal were notattenuated. In addition, once in a state in which brownout-preventionattenuation is taking place, predictive control state machine 140 may,based on such status information, determine whether to allow audiosignal amplitude to return to its non-attenuated level.

If the statuses of voltage monitor 122, supply response predictor 126,and battery impedance monitor 124 indicate that the battery is in aweakened state and audio amplitude detector 116 indicates that a highloading condition is about to occur, predictive control state machine140 may react by communicating an appropriate volume control signal tovolume controller 114, causing volume controller 114 to attenuate theaudio signal. Accordingly, by the time an audio signal potentiallycausing brownout is communicated to amplifier A1, it may be attenuatedto a level sufficiently low enough to prevent brownout.

FIG. 8 is a graphical representation of various signals versus time thatmay be present when a predictive brownout control system triggersattenuation of an audio signal to prevent brownout, in accordance withembodiments of the present disclosure. In FIG. 8, statuses of voltagemonitor 122, battery impedance monitor 124, and supply responsepredictor 126 are shown as indicating that a battery powering powersupply 10 is in a state weak enough such that it is not able to supportthe incoming audio signal AUDIO_IN without triggering a brownoutcondition once the audio signal propagates to amplifier A1 and loadsdown the battery powering power supply 10. This may cause predictivecontrol state machine 140 to generate an appropriate volume controlsignal to attenuate the audio signal responsive to analyzing informationreceived from audio amplitude detector 116 and power supply monitoring120 (e.g., responsive to an indication that an audio amplitude is abovea threshold level likely to cause a brownout condition). In someembodiments, the audio signal may be attenuated in amplitude steps ofVOL_(STEP1) for each time period t_(ATTACK) that the audio amplitude ofthe audio signal monitored by audio amplitude detector 116 remains abovethe threshold. Once the audio amplitude of the audio signal monitored byaudio amplitude detector 116 falls below the threshold (or anotherthreshold), the attenuation may be continued for a period t_(WAIT),after which the audio signal may be unattenuated in steps of VOL_(STEP2)for each time period t_(RELEASE) until returning to a normal state ofoperation with little or no audio attenuation.

FIG. 9 is a graphical representation of various signals versus time thatmay be present when a predictive brownout control system does nottrigger attenuation of an audio signal to prevent brownout, inaccordance with embodiments of the present disclosure. As shown in FIG.9, audio amplitude detector 116 indicates that an audio amplitude isbelow a threshold level likely to cause a brownout condition. However,in the scenario presented in FIG. 9, large audio amplitudes may only bepresent when voltage monitor 122, battery impedance monitor 124, and/orsupply response predictor 126 report that power supply 10 is capable ofhandling the loading caused by such audio amplitude. Such large audioamplitudes may not occur when a battery powering power supply 10 is in aweakened state, as indicated by statuses of voltage monitor 122, batteryimpedance monitor 124, and/or supply response predictor 126.Accordingly, under this set of circumstances, predictive control statemachine 140 may not cause attenuation of the audio signal.

FIG. 10 is a block diagram of selected components of an example audio IC1300 of a personal audio device, in accordance with embodiments of thepresent disclosure. The apparatus comprises a signal path having anaudio input configured to receive an audio input signal AUDIO_IN, anaudio output configured to provide an audio output signal V_(OUT), apower supply input configured to receive a power supply voltageV_(SUPPLY), and an attenuation block 1302. The attenuation block 1302 isconfigured to receive information indicative of one or more of thefollowing: 1) adaptive estimates of power supply conditions; 2)anticipated effects of power supply capacitance; and 3) at least onecondition of a complex load impedance. In response to determining fromthe received information that a portion of the audio output signal mayreach a maximum power threshold, the attenuation block 1302 may generatea selectable attenuation signal to reduce an amplitude of at least aportion of the audio output signal such that the signal path attenuatesthe audio input signal or a derivative thereof in order to preventbrownout prior to propagation to the audio output of the portion of theaudio input signal. The information indicative of adaptive estimates ofthe power supply conditions may comprise information regarding a voltagecomponent and a resistive component received from an adaptive batterymodel as described in FIG. 14. Such a voltage component and a resistivecomponent may be used to calculate the maximum power threshold.Similarly to FIG. 2 and as shown in FIG. 10, a digital audio source 1301(e.g., a processor, digital signal processor, microcontroller, testequipment, or other suitable digital audio source) may supply a digitalaudio input signal AUDIO_IN to an attenuation block 1302, which mayprocess digital audio input signal AUDIO_IN and provide such processedsignal to a digital-to-analog converter (DAC) 1303, which may in turnsupply an analog audio input signal V_(IN) to a power amplifier stage A2which may amplify or attenuate the audio input signal V_(IN) and providean audio output signal V_(OUT), which may operate a speaker, headphonetransducer, and/or a line level signal output. Although amplifier A2 isdepicted as a single-ended output generating a single-ended audio outputsignal V_(OUT), in some embodiments, amplifier A2 may comprise adifferential output, and may thus provide a differential audio outputsignal V_(OUT). A power supply 1304 may provide a power supply voltageV_(SUPPLY) to the power supply rail inputs of amplifier A2. Power supply1304 may comprise a charge pump power supply, a switching directcurrent-to-direct current converter, a linear regulator, or any othersuitable power supply.

It will be appreciated that IC 1300 may also comprise a predictivebrownout prevention system as previously described with reference toFIGS. 2 and 3.

In some embodiments, some or all of the functionality of the attenuationblock may be integral to the amplifier A2.

In FIG. 10 amplifier A2 may receive an audio input signal V_(IN) and mayapply a gain to increase signal amplitude usually in the form of avoltage. For audio and haptic amplifiers, high electrical currents mayresult Amplifier A2, when treated as a voltage amplifier, may typicallybehave as a scaling of audio input signal AUDIO_IN and may havefrequency effects, such as high-pass or low-pass filtering. Without lossof generality, amplifier A2 may receive the input voltage V_(IN) and mayprovide a gain to provide the amplified output voltage V_(OUT). Byknowing the load impedance, the current output of amplifier A2 is knownthrough Ohm's law (which applies also to complex impedances byconvolution). With both voltage and current known, attenuation block1302 may calculate the demand power of the amplifier as the product ofthe voltage and the current.

Amplifier A2 may be considered a power converter, which in reality willhave a less than perfect power conversion ratio. Attenuation block 1302may estimate the demand power of amplifier A2 and the efficiency ofamplifier A2, and from this estimate calculate the electricalcharacteristics at the amplifier input V_(IN), supply voltage V_(SUPPLY)and supply input current. Inserting the estimated amplifier demand powerinto a battery model with a capacitance element will mimic the behaviorof many real-world amplifier circuit configurations. In this example, asimple battery model is utilized; however, an adaptive battery model maybe used as described in reference to FIG. 14. The effects of supplyvoltage V_(SUPPLY) may then be modeled using audio input signalAUDIO_IN. Audio input signal AUDIO_IN may require attenuation to ensuresupply voltage V_(SUPPLY) remains above a threshold value, V_(THRESH).This attenuation may be calculated by attenuation block 1302.

For resistive loads, current and voltage may be in phase. However, forloads with reactance, such as a loudspeaker, the real power required fora particular frequency may be less than the product of theroot-mean-square voltage and current. This lower power requirement meansthat less attenuation is needed to ensure that the voltage (or current)threshold is met but not exceeded, and for example, allows for morepower to be delivered to the load, resulting in higher sound pressurelevel for speakers, and vibrational intensity for haptic systems.

For a given audio input signal AUDIO_IN, attenuation block 1302 mayutilize an estimate of the complex load impedance (Z_(LOAD)), and anestimate of pulse code modulated-to-V_(IN) transfer function (usually ascaling constant H), to calculate the demand power needed by amplifierA2 (P_(LOAD)) to amplify audio input signal AUDIO_IN using the followingequation:P _(LOAD)=Voltage*Current,  (1)

where Voltage=AUDIO_IN*H and Current=Voltage/Z_(LOAD). Due toinefficiencies of amplifier A2, the source power demanded by amplifierA2 to amplify the signal may be larger than the needed power P_(LOAD). Asource power P_(SRC) is set to be the source power demanded by amplifierA2. Source power P_(SRC) relates to P_(LOAD) by the efficiency parametern, which is between 0 and 1:P _(SRC) =P _(LOAD) /n  (2)

The estimate of the power demanded by amplifier A2 may be used, alongwith the estimates of the voltage component (V_(BATT)), the resistorcomponent (R_(BATT)), and the power supply capacitance (C_(BULK)) topredict the power supply voltage. In particular, for a given batterymodel, the maximum power that can be sourced while staying above avoltage threshold V_(THRESH) can be computed by solving the simultaneoussystem of equations relating the battery model to the power needed bythe amplifier.V _(SUPPLY) =V _(BATT)+SQRT(V _(BATT)^2−4*P _(SRC) *R _(BATT)))/2,  (3)where V_(BATT) is the battery model voltage and R_(BATT) is the batterymodel resistance. With this equation, the effects on the power supplyvoltage V_(SUPPLY) can be predicted as a function of the source powerdemanded by the amplifier, P_(SRC). If the power supply voltageV_(SUPPLY) is to stay above a threshold, then the maximum power allowedfor source power P_(SRC) can be determined. With the maximum allowedpower known, and the ability to estimate the power demand from the audioinput signal AUDIO_IN, the audio input signal AUDIO_IN may now beattenuated to satisfy the maximum power allowed condition.

In physical systems, capacitance effects can complicate the predictionof the power supply voltage V_(SUPPLY) due to the capacitancemomentarily supplying current rather than the battery. A simpleapproximation to the capacitance effect is to instead apply a low-passfilter to the estimated power demanded by the amplifier, and thislow-pass filter has a time-constant of approximately R_(BATT)*C_(BULK),where R_(BATT) is the resistance of the battery model and C_(BULK) isthe bulk capacitance in parallel to the power supply. Therefore, theinformation indicative of the anticipated effects of the power supplycapacitance (C_(BULK)) may be used to optionally apply a linear filteroperating on the predicated power supply voltage.

In order to ensure that the gain applied to the input audio signalactually protects the system from brownout, the gain may be appliedearlier than needed, i.e., at a voltage threshold above where brownoutwould actually occur, and maintained. In order to preserve causality,the audio input signal AUDIO_IN must be delayed, as well as its gaincomputation, in order to look ahead and apply the needed gain signalearlier.

With the power demand known, the maximum power allowed, i.e., themaximum power threshold for the audio input signal P_(MAX), can becalculated from equation (3) by replacing the V_(SUPPLY) with V_(THRESH)using the parameter estimates of the battery model and rearranging togive the following equation:P _(MAX)=(V _(THRESH) *V _(BATT) −V _(THRESH)^2)/R _(BATT),  (4)

wherein V_(THRESH) is the target minimum voltage value for power supplyvoltage V_(SUPPLY) that is allowed. Using the maximum allowed powerP_(MAX) and the estimate of the amplifier power demand P_(SRC), the gainG, required to satisfy the condition that the supply voltage V_(SUPPLY)must remain greater than V_(THRESH) is:

$\begin{matrix}{G = {\min( \frac{P_{{MA}\; X}}{P_{SRC}} )}^{\frac{1}{2}}} & (5)\end{matrix}$

With the gain value known, the protected signal amplitude is now known.This gain can be applied by the attenuation block to the broadbandsignal or to a summation of bandpass-filtered signals which comprisesthe broadband signal. For the bandpass-filtered signals, each band'sgain value may be adjusted, as long as the summation remains the same,which allows for a multiband compression ability.

The notion of attack and release times refers to the rate at which theapplied gain approaches the required gain to prevent brownout. Theseattack and release rates can be configured, typically in units ofdB/second.

In order to avoid excess current demands from the battery causing alow-voltage condition, capacitors may be added in parallel to powersupply 1304 in order to buffer momentary current spikes. Thesecapacitors may act as low-pass filters on the supply voltage V_(SUPPLY).In physical systems, this capacitance may also be augmented by parasiticeffects of connecting multiple components to power supply 1304 due totheir input capacitances, wiring topology, etc.

Amplifier A2, in the presence of this bulk capacitance, may pull currentfrom the capacitor and the battery, which means that there may be lessof a voltage drop as compared to the system without bulk capacitance. Byconsidering the effects of this bulk capacitance, the power demands onthe battery supply may be computed more accurately by the attenuationblock, and less attenuation on the audio input signal AUDIO_IN may beapplied while still ensuring that brownout conditions do not occur.

FIG. 11 is a plot diagram of power plotted against time that shows theeffects of bulk capacitance on a chirp signal, in accordance withembodiments of the present disclosure. FIG. 11 shows frequency of thechirp signal increasing with time. At higher frequencies, the bulkcapacitance may cause less of a power demand.

As described previously, FIG. 4 shows an example battery model. Thebattery model, for example as shown in FIG. 4 (e.g., and may be used aspower supply 10 in FIG. 2), comprises a Thevenin circuit of a batteryvoltage V_(BATT) with an output impedance Z_(OUT) or a series batteryresistance R_(BATT) which is a subset of output impedance Z_(OUT). Assystem current demand increases, available voltage decreases due toseries battery resistance R_(BATT), whereV_(SUPPLY)=V_(BATT)−R_(BATT)*I_(SUPPLY). Using only the V_(SUPPLY)signal, the battery voltage V_(BATT) and battery resistance R_(BATT)signals may be inferred for a changing I_(SUPPLY) signal (that is,changing current demand based on the load impedance), which in the caseof amplifier A1, the demand current I_(SUPPLY) would be the amplifierinput current I_(SUPPLY).

If there is no signal to amplify, which translates to no input current(I_(SUPPLY)=0), then supply voltage V_(SUPPLY) reveals the effectivebattery voltage V_(BATT). If other parts of the system, which areseparate from amplifier A2, draw current, such as a radio or a lightemitting diode (LED), there may be a supply voltage drop. From the pointof view of the battery model for amplifier A2, this supply voltage dropamounts to a reduced effective battery voltage V_(BATT).

When there is a signal for amplifier A2 which generates current draw,there may be a drop in supply voltage V_(SUPPLY). This supply voltagedrop may be proportional to battery resistance R_(BATT). By using anestimate of the power supply voltage V_(SUPPLY) for the given amplifieraudio input signal AUDIO_IN, a comparison may be made between the actualand predicted signals to adjust the estimates of battery resistanceR_(BATT). This may improve the accuracy of the battery model and hencemay result in less unnecessary attenuation of the audio signal.

Because of capacitance effects, group delay may be introduced into theactual supply voltage V_(SUPPLY) relative to the estimated supplyvoltage V_(SUPPLY) signal. This compensation may require knowing thegroup delay effects precisely, and slight phase errors may amplify orattenuate the difference between supply voltage V_(SUPPLY) and thepredicted or estimated supply voltage V_(EST) _(_) _(SUPPLY). In orderto avoid these phase effects, envelope predictions or estimations may beused.

If the battery model matches the real system, then the envelopes of theactual supply voltage V_(SUPPLY) and predicted or estimated supplyvoltage V_(SUPPLY) signals may be nearly identical. However, deviationsmay then reveal a mismatch between reality and the model and require acorrective response.

In FIG. 12, the bottom-side or bottom envelope of the supply voltageV_(SUPPLY) signals may provide information regarding the value ofbattery resistance R_(BATT). Hypothetically, if battery resistanceR_(BATT) were zero, then the bottom envelope and top envelope of thesupply voltage V_(SUPPLY) would be the same. However, as the batteryresistance R_(BATT) is not zero, the bottom envelope will be lower thanthe upper envelope as shown by the respective dotted lines V_(SUPPLY)and V_(EST) _(_) _(SUPPLY).

If the predicted supply voltage V_(EST) _(_) _(SUPPLY) does not fall asfar as the measured supply voltage V_(SUPPLY) signal, then batteryresistance R_(BATT) must be increased, and vice-versa, to correct forover-estimations in battery resistance R_(BATT).

In FIG. 13, the upper side or top envelope of supply voltage V_(SUPPLY)signals may be used for adapting the battery voltage V_(BATT) estimate.The peaks of supply voltage V_(SUPPLY) may provide the closest values tothe actual battery voltage V_(BATT) value. If the predicted peaks andactual peaks differ, then battery voltage V_(BATT) may need correction.

The presence of bulk capacitance confounds the battery voltage V_(BATT)and the battery resistance R_(BATT) adaptation, bulk capacitance for aparticular frequency can be simultaneously seen as a reduction inbattery voltage V_(BATT) and battery resistance R_(BATT). By includingbulk capacitance in the power estimation, battery voltage V_(BATT) andbattery resistance R_(BATT) estimates may become even more accurate,further reducing any unnecessary attenuation of the audio signal.

FIG. 14 illustrates an example of an adaptive battery model 1600 whichmay be included within or separate to attenuation block 1302 and/oramplifier A2. Battery model 1600 may comprise an envelope tracker 1601which may be configured to receive supply voltage V_(SUPPLY) and theestimate of supply voltage V_(EST) _(_) _(SUPPLY). Envelope tracker 1601may track the upper-side envelope of supply voltage V_(SUPPLY) in firsttracking block 1602 and compare the peaks of the resulting envelope tothe upper-side envelope of the predicted supply voltage upper-sideenvelope from second tracking block 1603. The comparison may be used toupdate the V_(BATT) value in update block 1604. Third tracking block1605 may track the bottom-side envelope of supply voltage V_(SUPPLY) andcompare the local minimums of the resulting envelope to the bottom-sideenvelope of the predicted supply voltage from fourth tracking block1606. The comparison is used to update the R_(BATT) value in updateblock 1607. The updated values of R_(BATT) and V_(BATT) may be providedto attenuation block 1302 for calculating the maximum power threshold.

As the audio input signal AUDIO_IN increases in amplitude, more powersupply current is needed to provide amplification. Too much power supplycurrent can lead to a brownout condition. The amplitude of audio inputsignal AUDIO_IN must be attenuated to avoid brownout. A dynamic rangecompressor and limiter can attenuate and limit the power supply current.

The embodiments of the present disclosure further: 1) provides anadaptive estimate of battery conditions; 2) anticipate effects of powersupply capacitance, and 3) use a complex impedance model of load (ratherthan a resistor) to determine needed power, since load reactance lowersneeded power (e.g., condition of load impedance).

This disclosure encompasses all changes, substitutions, variations,alterations, and modifications to the exemplary embodiments herein thata person having ordinary skill in the art would comprehend. Similarly,where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to theexemplary embodiments herein that a person having ordinary skill in theart would comprehend. Moreover, reference in the appended claims to anapparatus or system or a component of an apparatus or system beingadapted to, arranged to, capable of, configured to, enabled to, operableto, or operative to perform a particular function encompasses thatapparatus, system, or component, whether or not it or that particularfunction is activated, turned on, or unlocked, as long as thatapparatus, system, or component is so adapted, arranged, capable,configured, enabled, operable, or operative.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding this disclosureand the concepts contributed by the inventor to furthering the art, andare construed as being without limitation to such specifically recitedexamples and conditions. Although embodiments of the present disclosurehave been described in detail, it should be understood that variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the disclosure.

What is claimed is:
 1. An apparatus for providing an audio output signalto an audio transducer, comprising a signal path comprising: an audioinput configured to receive an audio input signal; an audio outputconfigured to provide an audio output signal; a power supply inputconfigured to receive a power supply voltage; and an attenuation blockconfigured to: receive information indicative of adaptive estimates ofpower supply conditions including information regarding a voltagecomponent and a resistive component based on an adaptive battery modelthat is usable to generate a predicted power supply voltage, wherein theadaptive battery model is based on a Thevenin circuit having a Theveninvoltage component and a Thevenin resistor component, wherein values ofthe Thevenin voltage component and the Thevenin resistor component arebased on a power supply voltage signal of the adaptive battery model, anamplifier input to an amplifier, an estimate of a power supplycapacitance, and an estimate of a load impedance of the amplifier; andbased on the received information, in response to determining that aportion of the audio input signal equals or exceeds a maximum powerthreshold that is based on the voltage component and the resistivecomponent, generate a selectable attenuation signal to reduce anamplitude of the audio output signal such that the signal pathattenuates the audio input signal or a derivative thereof in order toprevent brownout prior to propagation to the audio output of the portionof the audio input signal.
 2. The apparatus of claim 1, furthercomprising: a predictive brownout prevention system configured toprevent brownout of the audio output signal, wherein the predictivebrownout prevention system is configured to: receive informationindicative of an amplitude of the audio input signal; receiveinformation indicative of a condition of the power supply; determinefrom the information indicative of the amplitude of the audio inputsignal and the information indicative of the condition of the powersupply whether a brownout condition exists; and in response todetermining that a brownout condition exists, instruct the attenuationblock to generate the selectable attenuation signal.
 3. The apparatus ofclaim 1, wherein the audio input includes a plurality of audio inputs,and the audio output includes a plurality of audio outputs.
 4. Theapparatus of claim 1, wherein the attenuation block is furtherconfigured to receive additional information indicative of one or moreof: anticipated effects of power supply capacitance; and at least onecondition of a complex load impedance.
 5. The apparatus of claim 4,wherein the selectable attenuation signal is further based on theadditional information.
 6. The apparatus of claim 1, wherein theadaptive battery model is configured to provide an estimate of powerdemanded by the amplifier, wherein the estimate of the power demanded bythe amplifier is based on an output of the amplifier and the loadimpedance of the amplifier.
 7. The apparatus of claim 6, wherein thepredicted power supply voltage is based on the estimate of the powerdemanded by the amplifier and estimates of the Thevenin voltagecomponent, the Thevenin resistor component, and the power supplycapacitance.
 8. The apparatus of claim 1, wherein the attenuation blockis further configured to receive information indicative of anticipatedeffects of the power supply capacitance that is usable to apply a linearfilter to the predicted power supply voltage.
 9. The apparatus of claim1, wherein the adaptive battery model is configured to track localmaxima by: applying a first envelope detector for detecting a firstenvelope to the power supply voltage signal; and applying a secondenvelope detector for detecting a second envelope to the predicted powersupply voltage.
 10. The apparatus of claim 9, wherein an estimate of theThevenin voltage component is adjustable based on a difference betweenthe first and second envelopes.
 11. The apparatus of claim 1, whereinthe adaptive battery model is configured to track local minima by:applying a first envelope detector for detecting a first envelope to anegative of the power supply voltage signal; and applying a secondenvelope detector for detecting a second envelope to a negative of thepredicted power supply voltage.
 12. The apparatus of claim 11, whereinan estimate of the Thevenin resistor component is adjustable based on adifference between the first and second envelopes.
 13. The apparatus ofclaim 1, wherein the attenuation block is further configured to receiveinformation indicative of at least one condition of a complex loadimpedance that is usable to reduce the selectable attenuation signal inresponse to determining a high reactance in the complex load impedance.14. The apparatus of claim 2, wherein the signal path further comprisesa buffer configured to delay propagation of the audio input signal tothe audio output.
 15. A method for providing an audio output signal toan audio transducer, comprising: receiving information indicative of anamplitude of an audio input signal; receiving information indicative ofa condition of a power supply of a signal path having an audio input forreceiving the audio input signal and an audio output for providing theaudio output signal; receiving information indicative of adaptiveestimates of power supply conditions including information regarding avoltage component and a resistive component received from an adaptivebattery model, wherein the adaptive battery model is based on a Thevenincircuit having a Thevenin voltage component and a Thevenin resistorcomponent, wherein values of the Thevenin voltage component and theThevenin resistor component are based on a power supply voltage signalof the adaptive battery model, an amplifier input to an amplifier, anestimate of a power supply capacitance, and an estimate of a loadimpedance of the amplifier; using the adaptive battery model to generatea predicted power supply voltage; and based on the received information,in response to determining that a portion of the audio input signalequals or exceeds a maximum power threshold that is based on the voltagecomponent and the resistive component, causing attenuation of the audioinput signal or a derivative thereof to reduce an amplitude of the audiooutput signal in order to prevent brownout prior to propagation to theaudio output of the portion of the audio input signal.
 16. The method ofclaim 15, further comprising preventing brownout of the audio outputsignal by: determining from the information indicative of an amplitudeof the audio input signal and the information indicative of a conditionof the power supply whether a brownout condition exists; and in responseto determining that a brownout condition exists, causing the attenuationof the portion of the audio input signal.
 17. The method of claim 15,wherein the audio input includes a plurality of audio inputs and theaudio output includes a plurality of audio outputs.
 18. The method ofclaim 15, further comprising receiving additional information indicativeof one or more of: anticipated effects of power supply capacitance; andat least one condition of a complex load impedance.
 19. The method ofclaim 18, wherein the attenuation is further based on the additionalinformation.
 20. The method of claim 15, further comprising: computing,by the adaptive battery model, an estimate of power demanded by theamplifier from an output of the amplifier and the load impedance of theamplifiers to arrive at a product of voltage and current; and providing,by the adaptive battery model, the estimate of the power demanded by theamplifier.
 21. The method of claim 20, further comprising predicting thepower supply voltage using the estimate of the power demanded by theamplifier, along with estimates of the Thevenin voltage component, theThevenin resistor component, and the power supply capacitance.
 22. Themethod of claim 15, further comprising receiving information indicativeof anticipated effects of the power supply capacitance that is usable toapply a linear filter operating on the predicted power supply voltage.23. The method of claim 15, further comprising: applying a firstenvelope detector to the power supply voltage and a second envelopedetector to the predicted power supply voltage; outputting a firstenvelope from the first envelope detector for tracking local maximums inpower supply voltage; and outputting a second envelope from the secondenvelope detector for tracking the local maximums in the predicted powersupply voltage.
 24. The method of claim 23, wherein a difference betweenthe first envelope and the second envelope is used to adjust an estimateof the Thevenin voltage component.
 25. The method of claim 15, furthercomprising: applying a first envelope detector to a negative of thepower supply voltage signal and a second envelope detector to a negativeof the predicted power supply voltage; outputting a first envelope fromthe first envelope detector for tracking valleys and local minimums inthe negative of the power supply voltage; and outputting a secondenvelope from the second envelope detector for tracking minimums in thepredicted power supply voltage.
 26. The method of claim 25, wherein adifference between the first envelope and the second envelope is used toadjust an estimate of the Thevenin resistor component.
 27. The method ofclaim 15, further comprising receiving information indicative of atleast one condition of a complex load impedance that is usable to reducethe attenuation in response to determining a high reactance in thecomplex impedance.
 28. The method of claim 16, further comprisingdelaying propagation of the audio input signal to the audio output for aduration sufficient to permit attenuation of the audio input signal orthe derivative thereof before the portion of the audio input signalhaving the brownout condition propagates to the audio output.
 29. Anapparatus for providing an audio output signal to an audio transducer,comprising a signal path comprising: an audio input configured toreceive an audio input signal; an audio output configured to provide anaudio output signal; a power supply input configured to receive a powersupply voltage; and an attenuation block configured to: receiveinformation indicative of: 1) adaptive estimates of power supplyconditions including information regarding a voltage component and aresistive component based on an adaptive battery model that is usable togenerate a predicted power supply voltage; and 2) anticipated effects ofpower supply capacitance usable to apply a linear filter to thepredicted power supply voltage; and based on the received information,in response to determining that a portion of the audio input signalequals or exceeds a maximum power threshold, generate a selectableattenuation signal to reduce an amplitude of the audio output signalsuch that the signal path attenuates the audio input signal or aderivative thereof in order to prevent brownout prior to propagation tothe audio output of the portion of the audio input signal.
 30. Theapparatus of claim 29, further comprising: a predictive brownoutprevention system configured to prevent brownout of the audio outputsignal, wherein the predictive brownout prevention system is configuredto: receive information indicative of an amplitude of the audio inputsignal; receive information indicative of a condition of the powersupply; determine from the received information whether a brownoutcondition exists; and in response to determining that a brownoutcondition exists, instruct the attenuation block to generate theselectable attenuation signal.
 31. The apparatus of claim 30, whereinthe signal path further comprises a buffer configured to delaypropagation of the audio input signal to the audio output.
 32. Theapparatus of claim 29, wherein the audio input includes a plurality ofaudio inputs, and the audio output includes a plurality of audiooutputs.